Expressions

Expressions are path or formulas to access and modify your document data. Based on JSON path article (http://goessner.net/articles/JsonPath/), LiteDB support a near syntax to navigate in a single document. Path always returns an IEnumerable<BsonValue> in any case.

BsonExpression are the class that parse a string expression (or path) and compile into a LINQ Expression to be fast evaluate by LiteDB. Parser uses

Path starts with $: $.Address.Street
Int values starts with [0-9]*: 123
Double values starts with [0-9].[0-9]: 123.45
Strings are represented with a single ': 'Hello World'
Null are just null
Bool are represented using true or false keyword.
Functions are represented with FUNCTION_NAME(par1, par2, ...): LOWER($.Name)

Examples:

$.Price
$.Price + 100
SUM($.Items[*].Price)

Expressions can be used in your database to:

Create an index based on an expression: db.EnsureIndex(x => x.Name, "LOWER($.Name)")
Query documents inside a collection based on expression: Query.EQ("SUBSTRING($.Name, 0, 1)", "T")
Update field only data
Select command in shell: db.mycol.select $.Name, ARRAY(UPPER($.Books[*].Title)) AS titles where $.Name startswith "John"

Path

$ - Root
$.Name - Name field
$.Name.First - First field from a Name subdocument
$.Books - Returns book array value
$.Books[0] - Return first book inside books array
$.Books[*] - Return all books inside books array
$.Books[*].Title Return all titles from all books
$.Books[-1] - Return last book inside books array

Path also support expression to filter child node

$.Books[@.Title = 'John Doe'] - Return all books where title is John Doe
$.Books[@.Price > 100].Title - Return all titles where book price are greater than 100

Inside array, @ represent current sub document. It's possible use functions inside expressions too: